In cold-rolling of steel sheet, from the viewpoints of stabilization of the rolling operation, the shape and surface quality of the product, prevention of seizure, the roll lifetime, etc., it is necessary to maintain the friction coefficient between the rolled material (steel sheet) and the work rolls at a suitable value. To obtain a suitable friction coefficient, a lubricating oil suitable for the grade, dimensions, and rolling conditions of the rolled sheet is selected and supplied at the inlet side of the rolling stand to the rolled material or rolls.
At the cold-rolling of a steel sheet, in general emulsion lubrication is used. To obtain a suitable friction coefficient, a model is used to control the emulsion supply rate or emulsion concentration.
As methods for controlling lubrication by a model, there are:
(1) The method of estimating and controlling the supply rate of the seizing limit from a constant existing for each rolling condition, concentration, rolling rate, etc. (for example, see Japanese Patent Publication (Kokai) No. 2002-224731),
(2) The method of determining the positions of the lubricating oil supply nozzles by considering the time required for oil-water separation at the time the lubricating oil plates out on the steel sheet etc. (phase transition time) (for example, see Japanese Patent Publication (Kokai) No. 2000-094013), etc.
In the past, it was not possible to estimate or measure the oil film thickness at the time of emulsion lubrication. It was possible to arrange an oil film thickness meter at the outlet side of the rolling stand to measure the oil film thickness at the outlet side of the rolling stand, but it was not possible to learn the oil film thickness directly under the roll bite at a certain time. As a result, with the above conventional lubricating method, it was not possible to obtain a suitable oil film thickness right under the roll bite and not possible to control lubrication with a high precision.
Therefore, regarding the above method (1), since it is for the prediction of the seizing limit, use is not possible at a low speed. There is, therefore, room for improvement of the specific oil consumption in the low speed region. Further, regarding the above method (2), phase transition time is required for plateout of the emulsion lubricating oil. Setting the positions of the lubricating oil supply ends considering the phase transition time is, it is true, effective, but the method of determining the phase transition time is not fixed, therefore there is the problem that the positions cannot be accurately determined.